A laser usually consists of a volume filled with a light amplifying medium (gas, liquid, or solid) surrounded by a pair of parallel mirrors which cause the light to be repeatedly reflected through the amplifier. The two mirrors and the space between them are referred to as an optical cavity and the light trapped between them oscillates in the form of standing waves at frequencies, f.sub.N =Nc/2nd where c is the speed of light, n is the refractive index of the material between the mirrors, d is the distance between the mirrors and N is an integer.
Usually the amplifying medium is capable of amplifying a certain range of light frequencies called the "gain bandwidth", .DELTA.F, within which there may be several oscillating cavity frequencies, f.sub.N. Each of these oscillating frequencies is called a laser mode, and when more than one of these modes is oscillating and being emitted by the laser it is said to be in "multi-mode oscillation".
Short cavity dye lasers (SCDL) are known in the art and have been described by us in several articles: "Tunable Blue Picosecond Pulses From A Dye Laser," Applied Physics Letters 31, 389-391 (1977); "A Tunable Dye Laser In The 400-500 nm Range for Picosecond Spectroscopy," Proceedings of the Society of Photo-Optical Instrumentation Engineers 113, 25-34 (1977); "Short-Cavity Picosecond Dye Laser Design, Applied Optics 18, 532-535 (1979). FIG. 2 of the last article, a cross-section of a prior art SCDL, is presented herein as FIG. 1 for purposes of describing the basic operation of a SCDL.
Briefly, as shown in FIG. 1, the short cavity dye laser comprises a pair of mirrors 146 and 148 separated by a fraction of a millimeter, with the space between completely filled with a liquid organic dye solution 168 which serves as the light amplifying medium. Since the frequency separation of adjacent cavity mode frequencies is .DELTA.f=c/2nd, a short cavity length, d, results in a large mode spacing .DELTA.f. When .DELTA.f is greater than the gain bandwidth .DELTA.F, only a single cavity mode frequency is amplified and the laser is said to be in "single mode oscillation".
The laser is excited (pumped) by a short duration light pulse (6 to 300 ps) from another laser (e.g., Nd: glass, Nd: YAG, nitrogen) which is focused into the dye through one of the mirrors 146. The extremely short cavity length has a second benefit; it causes the dye laser to emit a light pulse which is of shorter duration than the pumping pulse. For example, when the SCDL is pumped with a 20 ps pulse, it emits a pulse of about 8 ps. Thus the laser is a picosecond (10.sup.-12 seconds) short cavity dye laser.
When the cavity length is varied slightly, the output frequency of the SCDL is also varied. This variation of output frequency (i.e., variable color) is referred to as "tuning" the laser.
In FIG. 1, the cavity length is adjusted by three micrometers 126, only one of which 126a is shown. The micrometers 126 bear against ball bearings 154 mounted in a support ring 130 supporting the output mirror 148. The input mirror 146 is mounted to another support ring 128 rigidly attached to the SCDL frame 124 by three rods 144, only one of which 144a is shown. An O-ring 150 is mounted between the support rings and surrounds the edge of the mirrors. The micrometers 126 push the output mirror-ring assembly 148-130 against the input mirror-ring assembly 146-128. By adjusting the micrometers 126, the amount of O-ring compression and thus the optical cavity-dye cell length may be varied. The O-ring 150 serves to seal the sides of the dye cell and acts as a preloaded spring against which the micrometers press.
Accurate cavity adjustment is difficult to achieve and maintain with the manually adjusted prior art SCDLs. As a result, it has been difficult to accurately tune the SCDL and to maintain tuning once achieved. As an additional result, it has been difficult to produce a rugged and reliable SCDL. It has also been difficult to manually tune a SCDL in "single mode oscillation" (single frequency or color) with the mode of operation found in prior art SCDLs.
Accordingly, it is the principal object of the present invention to accurately tune a SCDL and to maintain such accurate tuning.
It is an additonal object of this invention to allow extremely short cavity lengths in a SCDL.
It is a further object of this invention to achieve single mode oscillation in a SCDL.
Yet another object of this invention is to ruggedize a SCDL.
A further object of this invention is to continuously tune a SCDL within the lasing bandwidth of a typical dye gain curve.